The invention relates to a device for conveying coating powder out of a powder container.
In particular, the invention relates to a device for conveying powder or powdered material, particularly coating powder, whereby a powder pump is used to suction powder out of a powder container through a powder conveying tube fluidly connected or connectable to the powder pump.
The powder pump can for example be realized in the form of an injector having a carrier gas connector fluidly connected or connectable to a carrier gas line for conveying carrier gas, particularly conveyor air. The injector preferably further comprises a dosing gas connector connected or connectable to a dosing gas line for the regulated supplying of dosing gas, particularly dosing air, wherein the carrier gas is fed to the injector such that an area of negative pressure forms in the injector.
The invention further relates to a powder supply device for a powder coating system, wherein the powder supply device comprises at least one powder conveying device of the above type as well as at least one powder container.
Lastly, the invention further relates to a method for cleaning a powder conveying device of the above type.
Powder manufacturers supply spray coating powder to spray coating companies in powder containers, whereby such powder containers are frequently bags of powder. The spray coating companies thereafter use a spray coating system to spray the coating powder onto the objects to be coated. Heat is then applied to fuse the coating powder to the coated objects.
Powder bags, in which spray coating powder is commonly delivered to spray coating companies, can have the usual shape of a bag. They can also be formed from a plastic tube sealed at the top and bottom which is then cut open at the top in order to discharge the coating powder.
During the time coating powder is being drawn from them in the spray coating system, the powder bags often stay in the dimensionally stable carton in which the powder manufacturer supplied the respective powder bag.
Powder conveying devices are used in order to conduct coating powder out of the powder container of a spraying device. Such a powder conveying device normally comprises a powder pump, such as for example an injector, as well as a powder conveying tube connected to the intake connector of the powder pump, by means of which coating powder from the powder container is supplied to the spraying device for the spray coating of objects.
The principle behind injectors for pneumatically conveying coating powder from a powder container to a spraying device is generally known from e.g. powder coating technology. Spraying devices to which coating powder can be pneumatically conveyed by means of injectors can take the form of manually operable guns or automatically controlled sprayers. As shown by the U.S. Pat. No. 3,521,815 A, U.S. Pat. No. 4,802,625 A and U.S. Pat. No. 4,788,933 A printed publications, spraying devices can have varying configurations depending on the desired spraying method.
Spraying devices are known from the latter two cited printed publications which, in addition to a powder/gas flow, can also be fed cleaning gas which flows over electrodes to electrostatically charge the coating powder and thereby clean said electrodes and keep them free from powder deposit impurities. The high voltage for the electrodes can be produced in known manner by a high voltage generator incorporated into the spraying device or by an external high voltage generator. The high voltage of the high voltage generator creates a grounded electrostatic field between the electrodes and an object to be coated along which the coating powder particles flow from the spraying device to the object.
In order to achieve a constant flow rate for the powder/air mixture, the airspeed in the fluid lines, thus in particular in the powder conveying tubes, preferably needs to have a value between 10 to 15 m/s. At lower fluid line airspeed, powder conveyance becomes erratic; pulsation occurs in the powder/air mixture which continues through to the powder outlet on the spraying device. Higher airspeeds seriously impede the electrostatic depositing of coating powder on the object to be coated because they are coupled with the risk of blowing off the powder which has already settled onto the object.
The amount of powder fed to the spraying device is increased or decreased depending on the requirements of the coating process. One value from practical experience for the amount of powder supplied per unit of time is 300 g/min. When the amount of powder supplied per unit of time needs to be reduced, first the pressure at which the conveyor air is supplied to the injector is reduced. Doing so thus also reduces the flow rate of the conveyor air in the fluid lines. However, the total volume of air cannot become too low nor exceed a maximum value. In order to offset the reduction in the amount of air; i.e. so as to restore the airspeed to at least 10 m/s while maintaining the reduced powder output, more dosing air is supplied to the injector. The known operation of the injector is as follows:
The conveyor air produces a negative pressure in the injector which causes coating powder to be sucked out of a powder container, get picked up by the conveyor air and conducted through fluid lines of the spraying device. By changing the pressure, and thus also the amount of conveyor air, the amount of coating powder supplied per unit of time can be regulated. Since the conveying capacity is a function of the magnitude of the negative pressure created by the conveyor air in the injector, at constant or variable conveyor air, the conveyor air can also be regulated by introducing dosing air into the negative pressure area of the injector in order to change the magnitude of the negative pressure according to the desired powder feed rate. This means that the powder feed rate is not dependent solely on the volume of conveyor air but rather the difference of the conveyor air minus the dosing air. However, for the reasons given above, the total volume of air conveying the coating powder must remain constant during a given coating procedure.
A pneumatic powder conveying device of the above type; i.e. a device comprising a powder conveying tube and at least one powder pump, e.g. in the form of an injector, is particularly suited to supplying powder to powder coating systems which electrostatically spraycoat objects with powder in which fresh coating powder (also called “fresh powder” in the following) and reclaimed coating powder as applicable (also called “recovery powder” in the following) are in the powder container and are conveyed by a pneumatic powder conveying device of a spraying device of the type cited at the outset. As indicated above, the spraying device can for example be a manual gun or an automatic gun.
WO 01/15813 A2 discloses a device for conveying coating powder out of a powder container having a powder conveying tube with a powder inlet via which coating powder from a powder container can be fed to the powder conveying tube by suction, and a powder conveying tube retainer to hold the powder conveying tube, wherein the powder conveying tube is selectively displaceable relative to the powder conveying tube retainer in the powder conveying tube's longitudinal direction, and wherein for the purpose of cleaning the powder conveying tube, the powder inlet of the powder conveying tube can be closed and a compressed air line can be connected to the powder conveying tube, wherein the powder conveying tube retainer has a guide region with an opening arranged coaxially and concentrically to the longitudinal axis of the powder conveying tube through which at least part of the powder conveying tube extends.